Every day we see cars on the road, appliances in our homes, and other metal decorative objects, and we, most likely, give no attention to how they stay looking good for so long. One reason for their resistance to corrosion is that they went through a process of electrodeposition of electrocoat, or ecoat, on them. For instance, on automobiles, no other paint layer protects the metal exterior like ecoat. Anyone can formulate an electrocoat bath as long a few steps are followed. In order to better understand the formulation of ecoat baths, a brief history of electrocoat development and ecoat’s “green” aspect are needed.
Ford Motor Company did the majority of the development of automotive electrocoat in 1959 (DuPont Corporation). The first type of electrocoat developed was anodic electrocoat. In the anodic process, the part being coated is the anode, having a positive charge. Negatively charged ions in the electrocoat bath are attracted to the positively charged part and deposit on the surface. Ford’s Wixom Assembly Plant was the first automotive plant to have this type of electrocoat process in 1963 (DuPont Corporation).
The other type of electrocoat process, introduced in 1971, is cathodic (DuPont Corporation). In the cathodic process, the part being coated is negatively charged and positive ions in the bath are attracted and coated on the surface of the part. The cathodic process is superior to the anodic process because it offers far better corrosion resistance. It is for this reason that during the 1970’s work was done on perfecting this process. The first automotive plant to use this process of electrocoating was the Ford Oakville Assembly Plant in 1977 (DuPont Corporation). Anodic electrodeposition is still used on mainly household appliances that do not require the corrosion resistance that automobile require. In the early 1990’s a movement to eliminate lead from the electrocoat formulations took place (DuPont Corporation). As a result of the electrocoat baths being lead-free, the little waste generated at the automotive plants, from the electrocoat process, could be deposed in unregulated waste streams. In the latter part of the 1990’s and early 2000’s lower or zero HAP’s, hazardous air pollutant, solvent ecoats were developed (DuPont Corporation). Today, nearly 99% of automobile produced worldwide have cathodic electrocoat applied to them (Electrocoat.org).
The reason little waste is generated at automotive plants from the electrocoat process is because the process is a closed loop. It is a closed loop because almost 100% of the paint solids from the electrocoat tank are recovered. This is accomplished by a series of stage rinse tanks following the deposition tank. The first stage is usually the cream coat stage. This stage is located over the weir, or end, of the electrocoat tank. Here the solids that did not deposit on the part are rinsed off by permeate. Permeate is water and other low molecular weight species separated from the ecoat bath by the use of ultrafiltration. Ultrafiltration uses a series of spiral wound membranes that allow the ecoat bath to flow between the layers of semi-permeable membrane. Then, by a pressure gradient, water and other small molecular weight species are separated through the porous membrane and collected in a permeate holding tank for use in other stages. The second stage is usually the first recirculated permeate spray zone followed by a permeate dip tank, the third stage. The fourth stage is usually the second recirculated permeate spray zone. The last stage is a virgin permeate spray zone.
All of the post rinses are either increasing in altitude, allowing a passive counterflow of permeates to the deposition tank, or pumped to each preceding tank in an active counterflow. As a result of these counterflow system, the solids that were not deposited on the part are collected and transferred back to the deposition tank in a closed loop process. The closed loop, the waterborne nature of ecoat, low or zero HAP’s, and lead free give ecoat its green appeal. Today, a typical ecoat bath will contain 1-3% solvent content which means less impact on the environment.
The formulation of an electrocoat bath is relatively simple if the following series of steps are followed. First, the size of the bath needed and its physical constants such as non-volatiles, or solids, and pigment-to-binder ratio, P/B, must be known. For the sample calculation, a 2000 gram bath will be built at 19% solids and a 0.25 P/B. An electrocoat bath consists of mainly three ingredients: resin emulsion, paste, and deionized water. The physical constants of solids and P/B will need to be known for the resin emulsion and paste as well. For our calculation, the resin emulsion will be 34% solid and have a P/B of zero. The paste will have a solid percentage of 60% and a P/B of 2.5. The physical constant for any bath can be found on data sheets given by the supplier.
The first step in formulating an electrocoat bath is to multiply the desires bath size by the desired solids of the bath. For our example, this would be grams non-volatile. The next step is to calculate the grams non-volatile needed of paste. This is accomplished by dividing the grams non-volatile of the bath by the product of one plus the desired P/B of the bath divided by the desired P/B of the bath. This number is then multiplied by the product of one plus the P/B of the paste divided by the P/B of the paste resulting in the grams non-volatile of paste needed for the bath formulation. In our example this would be gNV. Due to the fact that deionized water has no solids, the grams non-volatile of resin emulsion needed can be calculated by subtracting the grams non-volatile of the paste from the total grams non-volatile of the bath. In our example this results in 273.6 grams non-volatile of resin emulsion. The fourth step is dividing the grams non-volatile of the resin emulsion and paste by their respective solids to obtain the actual grams of resin emulsion and paste that must be added to formulate the bath. This results in 804.7g and 177.3g of resin emulsion and paste, respectively. Finally, the sum of grams of resin emulsion and paste are subtracted from the total grams of the bath to determine the balance of deionized water needed. For the sample formulation, this is g deionized water.
Now that the steps of formulation an electrocoat bath are known, a person can formulate any size bath they desire as long as they know the few physical constants that are required. After learning the green aspects of electrodeposition and the awesome corrosion resistance afforded by electrocoat, it is easy to see why cars do not rust as early as their ancestors did. The development of electrocoat system did not happen over night, but for the sake of our automobiles, appliances, and other decorative metal objects, thank goodness it did develop.
References
DuPont Corporation. Science of Automotive: History of Electrocoat. ocoat/historyElectrocoat.htmlAccessed on May 26, 2009.
Electrocoat.org. Consumers Guide to Electrocoating.
Accessed on May 26, 2009.